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How to make a SCART switching box?


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Old 22-07-2004, 00:55
Zut
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Im a long time reader first time poster. These forums are great

Im building an HTPC and I have two A/V outputs that I would like to be able to pump through a single scart plug, but I rather than using a switch I would like the connection to be switchable by the computer itself (its a long story!). I will probably use a bit on the parallel port, and a some perl to control it.

Does anyone know how I would go about building such a device. My electronics skill doesn't really extend much beyond soldering!! Is it a matter of relays, or transistors or what. And what kind of current/votage/etc would I need to account for.

I haven't yetdone much research for this, but I thought I ask you lot first!

Thanks very much
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Old 22-07-2004, 13:34
pmcmillan
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You could just buy one!!
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Old 22-07-2004, 14:07
oldie
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From oldie. Yes, you could just buy one, but what's the fun in that? Besides Zut wants to use his computer to control it. If he can write a simple program to generate a voltage on a port (I can't because I am not a computer whizz-kid) he can use this to drive a transistor to operate a big solenoid to bang the switch in, on a commercially available switcher, each time to change from one A/V circuit to the other. Alternatively 20 single pole changeover relays will do the same thing. The relays MUST be capable of reliably switching voltages below 3 volts at a current of a few microamps. There are solid state switches which will work but they must have a very low on resistance (less than a few ohms) and work up to, say, 3MHz. Care must be taken to avoid capacitance between the video signal circuits as this will cause deterioration of the picture. If I had this problem, I would use a commercially available switcher which used solid state switches, controlled by a single user operated switch. I would then use the aforementioned program to operate a relay whose contacts are wired across the switch contacts in the switcher.
QED
Good luck.
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Old 22-07-2004, 14:22
Brush Master
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Oldie, no disrespect but you're talking electro-mechanics, not eletronics. Today, integrated circuitry is commonly used

Search Maxim or Analog Devices for some low-loss, multi-channel, CMOS video switch. Once you found a model, look for their recommended circuit application paper, and you'd be dancing.
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Old 23-07-2004, 00:43
Zut
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WOW! Thanks for the replies

I think I'll go somewhere in between, unless ~40 transistors is too expensive in which case I'll try the switcher CMOS route!

I think I remeber seeing a spec. for the digibox SCART socket on this site too, so that will be quite handy.

I'll try to knock up a basic diagram to show what Im thinking about, bearing in mind the only thing I know about transistors is what I looked up this morning!

Thanks again
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Old 23-07-2004, 07:42
Brush Master
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You might find ProAV's site useful - they don't show your schematics, but pinouts, loads of background, signal levels and other relevant data:http://www.proav.de/data/cables/scart-pinout.html

On the subject of transistors, I'd say this is the wrong route: transistors have quite some tolerance, and you won't have tools or knowledge to compensate for these tolerances. Integrated technology tends to have less tolerance between the channels on the same chip, and all switches on the same chip show very similar drift due to the thermic coupling. It's the same piece of silicon, basically.

On the subject of schematics, I suggest you use Google one "scart switch schematics" or similar keyword combinations. There are so many schematics out there; your's (or something close enough for easy adaption) is bound to be out there.
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Old 23-07-2004, 13:52
oldie
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From oldie.
To Brush Master. OK, I know my reference to relays, solenoids and discrete transistors shows my age but that's all we had to work with in those byegone days! And I've got lots of them in my spares boxes. Thanks for the ProAv URL. I'll have a look at that when I get time.
To Zut. You may not need to switch all 20 (21 is screen & ground) SCART wires if you only have left and right audio and the video is composite. If you get a ready built switcher, check that it will switch correctly. They don't all switch all SCART wires.
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